Ingredients

• 1 tablespoon extra-virgin olive oil
• 1 large onion, thinly sliced
• Unsalted butter, room temperature
• 4 1/3-inch-thick slices country white bread or sourdough bread
• 4 ounces thinly sliced smoked ham, divided
• 3 ounces Gouda cheese, thinly sliced, divided
• 2 large handfuls frisée, torn into bite-size pieces (about 1 cup), divided

Instructions

1. Heat oil in large nonstick skillet over medium heat. Add onion; sauté 5 minutes. Reduce heat to medium-low; cover and cook until onion is very tender and golden, stirring frequently, about 25 minutes longer. Season with salt and pepper. Cool slightly.
2. Butter 2 bread slices. Place bread, buttered side down, on platter. Divide onion, ham, cheese, and frisée between bread slices. Top each with another bread slice; butter top slices.
3. Heat another large nonstick skillet over medium heat. Add sandwiches and cook until bread is golden brown and cheese melts, pressing occasionally with spatula, about 4 minutes per side. Cut sandwiches in half diagonally and serve immediately.